Method for controlling the footprint area of a tyre and tyre for vehicle wheels

ABSTRACT

A method for controlling the symmetry of the footprint area of a tire running on a straight trajectory with camber angle different from zero, includes the steps: reducing the contact pressure of the tire on the footprint area at an inner shoulder (in case of negative camber) or at an outer shoulder (in case of positive camber); and disposing any medium line of the tread band placed in correspondence with the footprint area substantially parallel to the ground. A tire and a wheel for motor-vehicles, wherein the medium line of the tread band and the rotation axis of the tire form an angle substantially equal in absolute value to the camber angle. A process for manufacturing such tires, wherein a green tire with symmetric outer profile is deformed during the vulcanizing and moulding step until a predetermined angle different from zero is formed between any medium line of the tread band and the rotation axis of the vulcanized tire.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/IB2013/054628, filed Jun. 5, 2013, which claims the priority ofItalian Patent Application No. MI2012A001097, filed Jun. 22, 2012, andthe benefit of U.S. Provisional Application No. 61/672,036, filed Jul.16, 2012, the content of each application being incorporated herein byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention has as object a method for controlling thefootprint area of a tyre and a tyre for motor-vehicle wheels.

Preferably, the present invention refers to road tyres of UHP (UltraHigh Performance) type and to competition tyres that equipmotor-vehicles capable of high drive performances with straight linespeeds even greater than 300 Km/h.

Description of the Related Art

A tyre for motor-vehicle wheels generally comprises a carcass structureassociated with a belt structure. A tread band is disposed at a radiallyexternal position with respect to the belt structure. The tread is theportion of the tyre that comes into direct contact with the road surfaceand exchanges therewith the forces that allow driving the car along thetrajectories set by the driver.

By radial half-section of a tyre mounted or non-mounted on a rim, it isintended a section of the tyre carried out along a radial plane, i.e.containing the rotation axis of the tyre (or of the tyre mounted on therim) divided into two portions of the aforesaid rotation axis.

By camber angle “β” it is intended the angle complementary to the angleformed between the rotation axis of the wheel and the axis perpendicularto the ground passing through the rotation centre of the hub. Such angleis conventionally negative if the wheel is tilted towards the car, or inother words if the lower part of the tyre that touches the ground isfurther away from the car than the upper tyre part.

By mid-line plane “P_(T)” of the tyre (green or vulcanised and moulded),it is intended the plane orthogonal to the rotation axis of the tyre andaxially equidistant from the axially external ends of the beads of thetyre.

By mid-line plane “P_(W)” of the wheel, it is intended the planeorthogonal to the rotation axis of the wheel and axially equidistantfrom the axially internal ends of the two housings for the beads (rimedges) of the rim on which the tyre is mounted.

When the tyre is mounted on the rim, the two planes must coincide(P_(T)≡P_(W)).

It is observed that by axial end portions of the tread band (which in anon-deformed tyre define two circumferences corresponding to thecircumferential edges of the tread band), it is intended in this contextthe axial end points of the tyre that touch the road surface when thetyre in operating conditions, not collapsed (mounted on a rim andinflated to the operating pressure), is overloaded with a load equal toabout double the nominal load.

By median circumferential line of the tread band, it is intended the setof points “M” (which in a non-deformed tyre define a circumference)equidistant from the two axial end portions of the tread band itself.The aforesaid line is a radially external line of the tread band.

By medium line “lm” of the tread band, it is intended the straight linethat, in a radial half-section of the tyre, joins the two axial endportions of the tread band.

By symmetry axis “S” of the tread band, it is intended the straight linethat, in a radial half-section of the tyre, divides the tread banditself into two equal mirrored parts.

By radial distance, it is intended the distance measured along adirection orthogonal to the rotation axis “X-X” of the tyre andintersecting said rotation axis.

The document EP 0 755 808 illustrates a tyre comprising a carcassstructure, which is extended around the tyre from one bead to the other,two sidewalls and a tread zone. When the tyre is mounted on a rim andinflated to a predetermined pressure, the tread zone is asymmetric andhas the point of maximum diameter of the tyre shifted in the axialdirection, with respect to the central line of the tyre section, towardsthe inner sidewall. The radial distance from the point of maximumdiameter of the outer sidewall is greater than the radial distance fromthe point of maximum diameter of the inner sidewall, in a manner suchthat the tread zone has an asymmetric profile.

The document JP2009126424 illustrates a tyre having a first treadprovided with a low rolling resistance, a second tread provided with ahigh “grip” and a third tread. With small camber angles, the first treadrests on the ground while the second and the third tread are separatedfrom the road surface. With high camber angles, the second and the thirdtread come into contact with the road surface.

SUMMARY OF THE INVENTION

The Applicant has observed that the tyres are often mounted on amotor-vehicle with a camber angle for the purpose of optimizing thebehaviour of the car itself.

More precisely, the Applicant has observed that, during the running ofthe car on a straight line, the geometry with camber angle differentfrom zero produces a footprint area with a non-uniform pressuredistribution in tyres having symmetric profile. For example, a negativecamber angle produces, during the straight line advancing of themotor-vehicle, an asymmetric footprint area relative to a planeperpendicular to the ground containing the advancing direction andpassing through the rotation centre of the hub and such to shift thepoint of application of the resulting force due to the contact pressuresbetween the tyre and the road surface towards the inner sidewall(motor-vehicle side) of the tyre.

The Applicant deems that such effect is mainly due to the fact that themedium line of the tread band is not parallel to the supporting roadsurface but is tilted. It follows that, with each revolution of thewheel, the internal portion (motor-vehicle side) of the footprint areais flattened and deformed more than the external portion and thisimplies a non-uniform wear of the tyre in the tread (greater towards theinternal portion), non-regular over time and a non-optimal behaviour,due to the inefficiency of the pressure distribution, which alsonegatively influences the behaviour on a bend. For such purpose, theApplicant has set the objective of improving the performances of thetyres. In particular, the Applicant has perceived the need to propose atyre for motor-vehicle wheels that during operation ensures a moreuniform wear of the tread, more regular over time, and which preventsthe performance level of the tyre from excessively degrading during thelifetime thereof. In particular, the Applicant has perceived theimportance to ensure a pressure distribution that is as uniform aspossible on the footprint area of the tyre during the running on astraight line while maintaining the positive effect of the camber angleon a bend.

The Applicant has finally found that, by manufacturing a tyre formotor-vehicle wheels in which the tread band—while having a symmetricform in a radial half-section of the tyre—is rotated relative to aconventional symmetric tyre by an angle equal to the camber angle withwhich it is mounted on a motor-vehicle, one obtains that the medium lineof the tread band placed in correspondence with the footprint area onthe ground is substantially parallel to the ground. It is thus possibleto make the deformation of the tyre at said footprint area uniform instraight line running. The geometry of the tyre is therefore optimisedas a function of the camber angle prescribed by the manufacturer of themotor-vehicle model on which the tyre will be mounted.

More specifically, according to a first aspect, the present inventionregards a method for controlling the symmetry of the footprint area ofat least one same tyre running on a straight trajectory comprising:

-   -   installing said at least one tyre on a motor-vehicle with a        camber angle different from zero;    -   disposing any medium line of a tread band of the tyre        substantially parallel to the ground, once it is in        correspondence with the footprint area so as to reduce the        contact pressure of the tyre on the footprint area at an inner        shoulder of the motor-vehicle, if the camber angle is negative,        or at an outer shoulder of the motor-vehicle, if the camber        angle is positive.

The Applicant deems that maintaining the medium line of the portion ofthe tread band that rests on the ground substantially parallel to theground, in any case employing a camber angle, allows obtaining:

-   -   on a straight line path, that the variation of the length of the        footprint area (measured in the vehicle advancing direction)        along the axial direction (orthogonal to the vehicle advancing        direction) is reduced and the pressure distribution is more        uniform with respect to that of a tyre with symmetric cross        section mounted with an optimal, camber angle different from        zero in relation to that model of motor-vehicle;    -   on a bend, the beneficial effect of the camber angle on the        external wheel(s), which more greatly supports the centripetal        force exerted by the road surface on the tyre in a direction        substantially transverse to the advancing direction, is still        maintained.

In accordance with a second aspect, the present invention is related toa tyre for motor-vehicle wheels, comprising:

-   -   a carcass structure including a pair of beads having the same        radial distance from a rotation axis of the tyre;    -   a tread band disposed at a radially external position with        respect to the carcass structure;    -   wherein, in each radial half-section of the tyre, the tread band        is symmetric to a symmetry axis thereof;    -   wherein, in each radial half-section, a medium line of the tread        band and the rotation axis of the tyre form a predetermined        angle different from zero.

In other words, by observing the tyre in the abovementioned radialhalf-section, the tread band and preferably also the belt structure anda crown portion of the carcass structure have a symmetric shape,relative to a symmetry axis thereof separate from the mid-line plane ofthe tyre, similar to that of a conventional symmetric tyre. The treadband and preferably also the belt structure and the crown portion of thecarcass structure are therefore rotated, preferably around a pointbelonging to one of these elements, by the abovementioned predeterminedangle. The tread band is circumferentially extended around the rotationaxis by defining a substantially frustoconical shape.

The Applicant deems that the particular claimed geometry of the tyreallows the mounting thereof on a motor-vehicle with a camber angledifferent from zero, at the same time maintaining the medium lineparallel to the support surface, in a manner so as to minimize thedeformation irregularity (intended as asymmetry of the distribution ofthe length of the footprint area) of the tyre at the footprint area.Indeed, on a straight line path or with the car stopped, the footprintarea and the pressure distribution along the axial extension of saidarea result substantially symmetric since there is no contribution given(in the tyres with symmetric profile with negative camber angle) by thegreater flattening of the inner sidewall (motor-vehicle side) withrespect to the outer sidewall due to the tilt (equal to the camberangle) of the medium line of the tread relative to the ground. Theslight residual asymmetry is only due to the non-alignment of the mediancircumferential line with respect to the projection orthogonal to theroad plane of the hub centre, at which the vertical force that hits thetyre is transmitted.

In accordance with a third aspect, the invention is related to a wheelfor motor-vehicles, comprising:

-   -   a rim including two housings for the beads having the same        radial distance from a rotation axis of the wheel;    -   a tyre mounted on the rim, inflated to an operating pressure and        comprising a tread band;    -   wherein, in each radial half-section of the wheel, the tread        band is symmetric to a symmetry axis thereof;    -   wherein, in each radial half-section, a medium line of the tread        band and the rotation axis of the wheel form a predetermined        angle different from zero.

In accordance with a fourth aspect, the invention is related to amotor-vehicle, comprising:

-   -   at least one wheel mounted with a predetermined camber angle        different from zero;    -   wherein the wheel comprises a rim and a tyre mounted on the rim        and inflated to an operating pressure;    -   wherein, in each radial half-section, a corresponding medium        line of the tread band and the rotation axis of the tyre form a        predetermined angle substantially equal to said predetermined        camber angle, such that any medium line of the tread band placed        in correspondence with the footprint area is substantially        parallel to the ground.

In other words, each medium line of the tread band placed incorrespondence with the footprint area forms, with the rotation axis ofthe wheel, an angle equal in absolute value to the camber angle, withopposite sign.

By angle (between the medium line and the rotation axis) opposite thecamber angle, it is intended that, by observing the wheel in a radialplane, the rotation direction to be imparted to the rotation axis inorder to make it parallel to the medium line in contact with the ground(i.e. placed in correspondence with the footprint area) is opposite therotation direction assumed by the wheel in order to be tilted by thepredetermined camber angle, or vice versa that the rotation direction tobe imparted to the medium line in contact with the ground in order tomake it parallel to the rotation axis is opposite the rotation directionnecessary for straightening the wheel and making it assume a zero camberangle.

In accordance with a fifth aspect, the invention regards a process formanufacturing tyres for motor-vehicle wheels, comprising:

-   -   building a green tyre including at least one carcass structure        having a pair of beads and a tread band disposed at a radially        external position with respect to the carcass structure;    -   wherein any transverse half-section of said green tyre has an        outer profile that is symmetric to a mid-line plane thereof;    -   vulcanising and moulding said green tyre;    -   wherein, during the vulcanising and moulding step, said tyre is        deformed until a predetermined angle different from zero is        formed between any medium line of the tread band and the        rotation axis of the vulcanised and moulded tyre.

The Applicant deems that the aforesaid process according to the presentinvention allows manufacturing tyres as described without an excessiveincrease of costs, since for the building of symmetric green tyres, i.e.having cross section with symmetric outer profile, equipment andprocesses can be used of conventional type. Special equipment must beused only for vulcanising and moulding.

The present invention, in at least one of the aforesaid aspects, canalso have one or more of the preferred characteristics that aredescribed hereinbelow.

Preferably, a median circumferential line of the tread band is shiftedtowards the outside of the motor-vehicle by a predetermined distancerelative to a mid-line plane of at least one wheel comprising said atleast one tyre.

This solution, while slightly accentuating the asymmetry of thefootprint area and of the pressure distribution in a straight line path,improves the behaviour on a bend since each outer tyre with respect tothe curve (on which most of the centripetal force is discharged thatacts on that axis of the car) is deformed and the footprint area assumesa configuration that further decreases the already slightly accentuatedstarting asymmetry (relative to the mid-line plane P_(W)) that ischaracteristic of straight line running.

In addition, such preferred solution furthermore increases (with respectto the increase already provided by the negative camber) the wheel trackof the motor-vehicle, conferring greater stability thereto.

Preferably, the medium line of the tread band of said tyre remainssubstantially parallel to the ground during running on a bend.

The footprint area on the ground and the efficiency of the pressuredistribution are therefore maximised both on a straight line path and ona bend.

Preferably, the symmetry of the footprint area is controlled duringrunning on a straight trajectory on a set of four tyres installed insaid motor-vehicle.

Preferably, the tyre according to the invention is a road tyre of UHP orcompetition type.

Preferably the tread band is slick.

Preferably the tread band is provided with a tread design.

More in detail, the present invention preferably refers tohigh-performance tyres that are dedicated to very powerful cars, or moregenerally to applications that involve high operating speeds and/orextreme driving conditions, such as tyres of UHP (Ultra HighPerformance) type or tyres used in sports, like races on tracks (withstraight line speeds even exceeding 300 Km/h). The performance of suchtyres benefits from the positive effect of the tyre and the wheelaccording to the invention, more than other less performing tyres mightbenefit.

Preferably said predetermined angle has a width wider than about 0.5°.

Preferably said predetermined angle has a width smaller than about 5°.

Preferably, said predetermined angle is comprised between about 0.5° andabout 5°.

Such angle is selected in the field of values usually used as camberangles for road or competition motor-vehicles.

According to a preferred embodiment of the tyre, axial end portions ofthe tread band are axially spaced apart the same distance from themid-line plane of the tyre.

This signifies that, relative to a conventional symmetric tyre, thetread band (and preferably also the belt structure and the crown portionof the carcass structure) is simply rotated around the point “i”identified by the intersection between the mid-line plane “P_(T)” of thetyre and the medium line “lm” of the tread band.

According to a different embodiment, a median circumferential line ofthe tread band is axially shifted by a predetermined distance relativeto the mid-line plane of the tyre.

This signifies that, relative to a conventional symmetric tyre, thetread band (and preferably also the belt structure and the crown portionof the carcass structure) in addition to being rotated is alsotranslated along an axial direction, i.e. a direction parallel to therotation axis.

Preferably, said predetermined distance is included between about 3 mmand about 30 mm.

Such range is equivalent to the transverse translation of a centre linepoint when a wheel (tyre mounted on rim and inflated to the operatingpressure) with 700 mm outer diameter passes from a zero camber to acamber angle of 0.5° as minimum and 5° as maximum, according to thesimplified formulaδy=Dex*β*π/360,where δy is the translation of the band, Dex the outer diameter of thewheel, and β the camber angle.

Preferably, axial end portions of the tread band have axial distancesdifferent from the mid-line plane of the tyre.

Preferably, the axial end portion that is axially the farthest from themid-line plane is also the one that is radially the farthest from therotation axis.

This signifies that, by observing the tyre in said radial half-section,a clockwise rotation of the tread band corresponds with a translation tothe left of the same tread band and an anti-clockwise rotation of thetread band corresponds with a translation to the right of the same treadband.

Preferably, the beads are symmetric relative to the mid-line plane.

The position and the shape of the beads are substantially equal to thatof a conventional symmetric tyre.

Preferably, the tyre has asymmetric sidewalls relative to the mid-lineplane.

The rotation and the translation of the tread band are mainly obtainedthrough a deformation of the beads. If the tread band is only rotated,one of the beads is radially compressed and the other radially extendedrelative to a symmetric tyre. If the tread band is also translated, oneof the beads is rotated towards the mid-line plane and the other isrotated away from said mid-line plane.

Preferably, a median circumferential line of the tread band is axiallyshifted by a predetermined distance relative to a mid-line plane of thewheel.

Preferably, the rim has housings for the beads that are symmetric to amid-line plane.

Preferably, the tyre mounted on the rim and inflated to the operatingpressure has asymmetric sidewalls relative to a mid-line plane.

Preferably, the camber angle is negative.

Such preferred solution is employed in particular for very powerfuland/or competition cars and ensures a better support on a bend and abetter straight line stability.

Preferably the width of the camber angle is wider than about 0.5°.

Preferably the width of the camber angle is smaller than about 5°.

Preferably, the camber angle is comprised between about −0.5° and about−5°.

Preferably, a median circumferential line of the tread band is axiallyshifted by a predetermined distance relative to a mid-line plane of thewheel.

Preferably, the median circumferential line is shifted towards theoutside relative to the motor-vehicle.

Preferably, an axial end portion of the tread band that is axially thefarthest from the mid-line plane is also the one that is radially thefarthest from the rotation axis.

The asymmetry of the wheels that results from the above-delineatedcharacteristics shifts the contact between the tyres and the roadtowards the outside, increasing the wheel track and at the same timeensuring the maximum contact area possible both on a straight line pathand on a bend.

Preferably, the rim has housings for the beads that are symmetric to themid-line plane.

The preferred rim is per se a conventional rim that does not require anymodification for receiving the tyre which gives rise to the wheelaccording to the invention.

Preferably, in a radial half-section of the tyre, the tread band issymmetric to a symmetry axis thereof.

Preferably, the tyre has asymmetric sidewalls relative to a mid-lineplane of the wheel.

Preferably, after vulcanisation and moulding, the tread band, seen in aradial half-section of the tyre, is symmetric to a symmetry axisthereof.

Vulcanisation and moulding deform the sidewalls, increasing the diameterof one of the circumferential edges of the tread band and reducing thediameter of the other, but preferably, in each radial half-section, thetread band remains substantially undeformed and preferably the beadsremain symmetric to the mid-line plane.

In a preferred embodiment, during vulcanisation and moulding, the treadband is axially shifted by a predetermined distance relative to themid-line plane of the tyre.

Preferably, during vulcanisation and moulding, one of the two axial endportions of the tread band is moved away axially from the mid-line planeand radially from the rotation axis while the other of said two axialend portions is axially moved close to the mid-line plane and radiallymoved close to the rotation axis.

The roto-translation of the tread band is obtained by means of arotation of the sidewalls (one towards the mid-line plane of the tyreand the other moving away) substantially around the respective beads.Relative to the green tyre, the extension of each of the sidewalls inthe abovementioned radial half-section (intended as length of the lineformed by the points equidistant from the axially internal face andaxially external face of each sidewall) varies very little, i.e. thesidewalls are not subjected, in moulding and vulcanising, to excessiveand damaging stretching or compression deformations.

Preferably, after vulcanisation and moulding, the tread band, seen ineach radial half-section of the tyre, remains substantially undeformed.

Preferably, after vulcanisation and moulding, the beads are symmetricrelative to the mid-line plane of the tyre.

Further characteristics and advantages will be clearer from the detaileddescription of preferred but not exclusive embodiments of a method forcontrolling the symmetry of the footprint area of at least one same tyrerunning on a straight trajectory with camber angle different from zero,of a tyre for motor-vehicle wheels, of a wheel for motor-vehicles, of amotor-vehicle and of a process for manufacturing tyres for motor-vehiclewheels in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Such description will be set forth hereinbelow with reference to the setof drawings, provided only as a non-limiting example, in which:

FIG. 1 shows a radial half-section of a wheel for motor-vehicles inaccordance with a first embodiment of the present invention;

FIG. 2 shows a radial half-section of a wheel for motor-vehicles inaccordance with a second embodiment of the present invention;

FIG. 3 schematically shows a motor-vehicle provided with the wheel ofFIG. 1 in a respective operating condition;

FIG. 3a illustrates the transverse distribution of the footprint contactpressures of the wheel of FIG. 3 calculated at the radial section of thetyre orthogonal to the road plane;

FIG. 4 schematically shows a motor-vehicle provided with the wheel ofFIG. 2 in a respective operating condition;

FIG. 4a illustrates the transverse distribution of the footprint contactpressures of the wheel of FIG. 4 calculated at the radial section of thetyre orthogonal to the road plane;

FIG. 5 schematically shows a motor-vehicle provided with a conventionalsymmetric wheel (belonging to the prior art) with the same camber angleof the wheels of FIGS. 3 and 4;

FIG. 5a illustrates the transverse distribution of the footprint contactpressures of the wheel of FIG. 5 calculated at the radial section of thetyre orthogonal to the road plane.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the abovementioned figures, a wheel for motor-vehicleswas indicated in its entirety with 1, such wheel comprising a tyre 2.

The tyre 2 has a carcass structure 3 which comprises at least onecarcass ply 3 a preferably internally covered by a layer of impermeableelastomeric material or so-called liner 4. Two annular anchoragestructures 5, each comprising a so-called bead core 5 a preferablybearing an elastomeric filler 5 b at a radially external position, areengaged with respective end flaps of the carcass ply or plies 3 a. Theannular anchorage structures 5 are integrated in proximity to zonesnormally identified with the name of “beads” 6, at which the engagementbetween the tyre 2 and a respective mounting rim 7 normally occurs,according to a rim diameter determined by the internal diameterdimensions of the annular anchorage structures 5. A belt structure 8normally comprising one or more belt layers 8 a is circumferentiallyapplied around the carcass ply or plies 3 a, and a tread band 9 iscircumferentially superimposed on the belt layers 8 a. Two sidewalls 10,each being extended from the corresponding bead 6 to a correspondingside edge of the tread band 9, are applied in laterally oppositepositions on the carcass ply or plies 3 a.

The rim 7, per se known, has a substantially cylindrical body 11provided with a radially external channel, on which the tyre 2 isplaced. The substantially cylindrical body 11 delimits an axiallyexternal housing 12 a (i.e. directed towards the outside of amotor-vehicle when the rim is mounted on said motor-vehicle) and anaxially internal housing 12 b (i.e. directed towards the motor-vehicleside when the rim is mounted on said motor-vehicle), one for each bead 6of the tyre 1, defined by respective annular slots that are radiallyexternal and symmetric relative to a mid-line plane “P_(W)” orthogonalto a rotation axis “X-X” of the wheel 1 (here substantially coincidingwith the rotation axis of the tyre 2 and indicated in the same manner).The rim 7 also comprises a body 13 that is radially internal, relativeto the substantially cylindrical body 11, in which devices are obtained(not illustrated and defined, for example, by holes and relative bolts)for coupling the wheel 1 to the hub of a motor-vehicle. In theillustrated embodiment, the radially internal body 13 is offset relativeto the mid-line plane “P_(W)” and shifted towards the axially externalhousing 12 a of the rim 7 in a manner so as to provide the necessaryspace inside the substantially cylindrical body 11 for placing the huband the braking devices (e.g. brake discs and callipers).

The radial distance “r” of each of the two axially external/internalhousings, respectively 12 a, 12 b, from the rotation axis “X-X”,measured along a diameter of the rim 7, is the same. The axial distance“x”, measured parallel to the rotation axis “X-X”, of each of the twoaxially external/internal housings, respectively 12 a, 12 b, from themid-line plane “P_(W)” is the same.

The two beads 6 of the tyre 1, each installed in a respective axiallyexternal/internal housing 12 a, 12 b, are also symmetric relative to themid-line plane “P_(W)” (or “P_(T)”). The radial distance “r” of each ofthe two beads 6 from the rotation axis “X-X”, measured along a diameterof the rim 7, is the same. The axial distance “x” of each of the twobeads 6, measured parallel to the rotation axis “X-X”, from the mid-lineplane “P_(W)” (or “P_(T)”) is the same.

When the tyre 2 is not mounted on the rim and when it is mounted on therim, inflated to the operating pressure but not subjected to externalstress forces, said tyre 2 has a non-symmetric geometry in a radialsection (FIGS. 1 and 2).

In both the embodiments illustrated in the enclosed figures—excludingthe beads 6—the carcass structure 3, the belt structure 8, the treadband 9 and the sidewalls 10 of the tyre 2 are asymmetric to the mid-lineplane “P_(W)” (or “P_(T)”).

In the first embodiment, illustrated in FIGS. 1 and 3, relative to aconventional symmetric tyre, the tread band 9 together with the beltstructure 8, and at a crown portion of the carcass structure 3, seen ina radial half-section of the tyre 2, are rigidly rotated around thepoint “i” of intersection between the straight line corresponding to themid-line plane “P_(W)” (or “P_(T)”) and the medium line “lm” of thetread band 9 by a predetermined angle “α”.

By crown portion of the carcass structure 3, it is intended the radiallyexternal portion thereof associated with the belt structure 8 and withthe tread band 9.

In the embodiment illustrated as an example in FIG. 1, suchpredetermined angle “α” is equal to about 3°. Preferably, suchpredetermined angle “α” is comprised between about 0.5° and about 5°.

In the abovementioned radial half-section (FIG. 1), the tread band 9together with the belt structure 8 and the crown portion of the carcassstructure 3 maintain a symmetry thereof relative to a symmetry axis “S”that is tilted by the same predetermined angle “α” relative to themid-line plane “P_(W)” (or “P_(T)”).

The symmetry axis “S” intersects a radially external surface of thetread band 9 at a point “M” belonging to the median circumferential lineof the tread band 9. The two axially opposite portions 9 a and 9 b ofthe tread band 9 have the same axial distance “Z”, measured along adirection parallel to the rotation axis “X-X” of the wheel 1, from themid-line plane “P_(W)” (or “P_(T)”). Said two opposite axial endportions 9 a and 9 b also have a radial distance “d1, d2”, measuredalong a diameter of the wheel 1, from the axially external/internalhousing provided on the rim 7. In particular, the radial distance “d1”of the axially external axial end portion 9 a from the respectiveaxially external housing 12 a is greater than the radial distance “d2”of the axially internal axial end portion 9 b from the respectiveaxially internal housing 12 b.

The medium line “lm” of the tread band 9 is therefore tilted (towardsthe axially internal housing 12 b of the rim 7) with respect to therotation axis “X-X” (or, as illustrated in FIG. 1, with respect tostraight lines parallel to the abovementioned rotation axis “X-X”) bythe predetermined angle “α”.

The maximum distance (or camber), measured parallel to the symmetry axis“S”, between the radially external surface of the tread band 9 and themedium line “lm” is situated at the symmetry axis “S” itself. In otherwords, the maximum thickness of the tread band 9 (which is placed at thesymmetry axis “S”) is situated at the median circumferential line and inproximity to the mid-line plane “P_(W)” (or “P_(T)”) of the wheel 1 orof the tyre 2.

The two-dimensional geometry just illustrated with reference to theradial half-section corresponds with a tyre 2 in which the theoreticalsurface that connects opposite circumferential edges (set of the pointsconstituting the two opposite axial end portions 9 a and 9 b) of thetread band 9 is a truncated cone. The truncated cone is tapered towardsthe axially internal housing 12 b of the rim 7.

The circumferential edge with smaller diameter is associated with asidewall 10 that is radially flattened with respect to that of acorresponding conventional symmetric tyre. The circumferential edge withgreater diameter is associated with the other sidewall 10 that isradially elongated with respect to that of a corresponding conventionalsymmetric tyre.

For example, the camber of the tread band 9 can be equal to about 1/30of the width of the tread band (distance between the two opposite axialend portions 9 a and 9 b). It follows that the radially external surfaceof the tread band 9 is only slightly curved and has a substantiallyfrustoconical shape.

The wheel 1 with the tyre 2 inflated to the operating pressure ismounted on the motor-vehicle “C” with a camber angle “β” different fromzero (FIG. 3). In the embodiment illustrated in FIG. 3, such camberangle “β” is negative and equal to about −3°. The absolute value of thecamber angle “β” is equal to the absolute value of the abovementionedpredetermined angle “α”.

Since the axially internal housing 12 b of the rim 7 is mounted directedtowards the motor-vehicle “C”, the truncated cone is tapered towards themotor-vehicle “C” itself. Given that the camber angle “β” is negativeand equal to the predetermined angle “α”, i.e. to half the angle at thevertex of the conical surface to which the truncated cone belongs, itresults that the medium line “lm” of the tread band 9 placed at theground is parallel to the ground itself (FIG. 3).

On a straight line path, the vertical force “F” that is discharged bythe hub on the rim and then on the ground through the tyre 2 (or viceversa the force that acts from the ground on the hub by means of thetyre 2 and the rim 7) generates a distribution of pressures on thefootprint area of the tyre 2. In FIG. 3a , the transverse pressuredistribution “P1” (along the axial extension) of the footprint area isillustrated according to the first embodiment of the invention.

Illustrated in FIG. 5a is the transverse pressure distribution “P3” ofthe footprint area of a wheel 1′ provided with a conventional symmetrictyre 2′ mounted on the motor-vehicle “C” with the same camber angle “β”with which the wheel 1 is mounted according to the invention of FIG. 3.In FIG. 5a , the rotation axis “X′-X′”, the medium line “l′m”, the setof the points “M′” equidistant from the two axial end portions of thetread band, the symmetry plane “P′_(W)” of the wheel 1 and the symmetryplane “P′_(T)” of the tyre 2′ according to the prior art are marked by aprime.

As can be observed, the transverse pressure distribution on the tyre 2of FIG. 3a is more uniform than that of the conventional tyre 2′ of FIG.5a . In particular, in the conventional tyre 2′ of FIG. 5 the transversedistribution of the footprint contact pressures “P3” is stronglyasymmetric due to the greater flattening of the internal side of thetyre 2′ due to the negative camber and to the respective medium linetilted relative to the road plane.

In the tyre 2 according to the invention of FIG. 3, the transversedistribution of the footprint contact pressures “P1” (FIG. 3a ) is onlyslightly asymmetric and such slight asymmetry is due only to thenon-alignment between the centre line of the footprint area (whichcorresponds to the median circumferential line of the tread band 9placed in correspondence with the footprint area) and the vertical linealong which the force “F” acts. The asymmetry of the pressuredistribution “P1” is reduced since the medium line “lm”, tilted withrespect to the rotation axis “X-X” by the predetermined angle “α” equalto the camber angle “β” (with opposite sign thereto), is parallel to theroad plane.

Considering an average pressure, intended integral of the transversepressure distribution “P1”, “P3” divided by the width “W1”, “W3” of thefootprint area, the peak pressure “P3max” of the distribution “P3”according to the prior art is about double the average pressure “P3av”of the same distribution. Instead, the peak pressure “P1max” of thedistribution “P1” according to the first embodiment is only about 4/3 ofthe average pressure “P1av”.

The second embodiment illustrated in FIGS. 2 and 4 is distinguished fromthe first embodiment since the tread band 9 together with the beltstructure 8 and the crown portion of the carcass structure 3, inaddition to being rigidly rotated the predetermined angle “α”, are alsotranslated along an axial direction towards the axially external housing12 a of the rim 7. The symmetry axis “S” of the tread band 9 of eachradial half-section is axially shifted by a predetermined distance “Dx”relative to the mid-line plane “P_(W)” (or “P_(T)”) of the wheel 1 or ofthe tyre 2. Also the median circumferential line is substantiallyshifted the same predetermined distance “Dx”. Preferably, suchpredetermined distance “Dx” is included between about 3 mm and about 30mm.

It follows that the axially external axial end portion 9 a is furtherboth from the mid-line plane “P_(W)” (or “P_(T)”) and from the rotationaxis “X-X” with respect to the axially internal axial end portion 9 b.In the illustrated embodiment, the axial distance “Z1” from the mid-lineplane “P_(W)” of the axially external axial end portion 9 a is greaterthan the axial distance “Z2” from the mid-line plane “P_(W)” of theaxially internal axial end portion 9 b.

The sidewall 10 corresponding with the axially external axial endportion 9 a is rotated around the respective bead 6 towards the outsideof the wheel 1, moving away from the mid-line plane “P_(W)” (or “P_(T)”)while the sidewall 10 corresponding to the axially internal axial endportion 9 b is rotated around the respective bead 6 towards the interiorof the wheel 1, moving close to the mid-line plane “P_(W)” (or “P_(T)”).

Also the wheel 1 with the tyre 2 inflated to the operating pressure andin accordance with the second embodiment is mounted on the motor-vehicle“C” with the camber angle “β” equal to about −3° and equal in absolutevalue to the predetermined angle “α” (FIG. 4).

The difference with respect to the wheel 1 of FIG. 3 lies in the factthat the tread band 9 is further from the motor-vehicle “C” than thepredetermined distance “Dx” while the positions of the rim 7, of the huband of the vertical line along which the force “F” acts are the same.

As can be observed, during the running on a straight line, thetransverse pressure distribution on the tyre 2 of FIG. 4a is moreuniform than that of the conventional tyre 2′ of FIG. 5a even if lessuniform than the tyre 2 of FIG. 3a . Instead, the peak pressure “P2max”of the distribution “P2” in accordance with the second embodiment isabout 3/2 the average pressure “P2av” (intended as integral of thetransverse pressure distribution “P2” divided by the width “W2”). Suchgreater but still limited asymmetry relative to the tyre 2 of FIG. 3a isdue to the greater non-alignment between the centre line of thefootprint area (which corresponds with the median circumferential lineof the tread band 9 placed in correspondence with the footprint area)and the vertical line along which the force “F” acts.

The asymmetric tyres 2 as described above are preferably manufactured bymeans of building a conventional green tyre with cross section withexternal profile that is symmetric to a mid-line plane “P_(T)” thereof.Preferably but not exclusively, the symmetric green tyre is built bymeans of assembly of respective components on a forming support.Subsequent to the building of the symmetric green tyre, a vulcanisingand moulding treatment is executed aimed to: determine the structuralstabilization of the tyre by means of cross-linking of the elastomericcompositions, conferring an asymmetric shape to the vulcanised tyre forexample as was previously described with reference to the twoillustrated embodiments, as well as imparting a desired tread design onthe same tyre and possible distinctive graphic signs at the sidewalls ofthe tyre.

For such purpose, the green tyre is introduced into a vulcanisationmould comprising portions arranged mutually adjacent which, once closed,delimit a vulcanisation and moulding cavity adapted to confer thedesired form to the vulcanised tyre 2.

In particular, in order to obtain the tyre 2 in accordance with thefirst embodiment of FIGS. 1 and 3, with vulcanisation and moulding, oneof the sidewalls 10 is radially compressed in order to reduce thediameter of the corresponding axial end portion 9 b of the tread band 9and the other sidewall 10 is radially extended in order to increase thediameter of the corresponding axial end portion 9 a of the tread band 9,in a manner so as to rotate said tread band 9 by the abovementionedpredetermined angle “α”.

In order to obtain the tyre 2 in accordance with the second embodimentof FIGS. 2 and 4, one of the sidewalls 10 in additionally to beingradially compressed is rotated around the respective bead 6 in order toreduce the diameter of the corresponding axial end portion 9 b of thetread band 9 and shift it towards the mid-line plane “P_(T)”. The othersidewall 10 is radially extended and rotated around the respective bead6 in order to increase the diameter of the corresponding axial endportion 9 a of the tread band 9 and to move it away from the mid-lineplane “P_(T)”, in a manner so as to rotate said tread band 9 by theabovementioned predetermined angle “α” and translate it theabovementioned distance “Dx”.

The tyres 2 described allow actuating a method for controlling thesymmetry of the footprint area of the tyre running on a straighttrajectory with camber angle different from zero, preferably on a set offour tyres 2 in a motor-vehicle “C”. In the conventional symmetric tyreswith camber angle different from zero, like that illustrated in FIG. 5,the distribution of the contact pressure is strongly asymmetric andshifted towards the shoulder of the tyre 2′ which results more flattenedmainly due to the tilt of the medium line “lm” of the tread band. If thecamber angle is negative, the more flattened shoulder is the innershoulder towards the motor-vehicle; if the camber angle is positive, themore flattened shoulder is the outer one.

The method provides for reducing the contact pressure of the tyre on thefootprint area at an inner shoulder, if the camber angle β is negative,or at an outer shoulder, if the camber angle β is positive, disposingany medium line “lm” of the tread band 9 placed in correspondence withthe footprint area substantially parallel to the ground (FIG. 3).

The method also preferably provides for shifting a mediancircumferential line of the tread band 9 towards the outside of themotor-vehicle “C” by a predetermined distance “Dx” relative to amid-line plane “PW” of said wheel 1 (FIG. 4). The medium line “lm” ofthe tread band 9 of the tyre 2 remains substantially parallel to theground during running on a bend, limiting the asymmetry of the footprintarea also in this operating condition.

The invention claimed is:
 1. A tyre for motor-vehicle wheels,comprising: a carcass structure comprising a pair of beads having a sameradial distance from a rotation axis of the tyre; and a tread banddisposed at a radially external position with respect to the carcassstructure, wherein the are symmetric relative to a mid-line plane of thetyre, wherein, in each radial half-section of the tyre, the tread bandis symmetric relative to a symmetry axis thereof, wherein, in eachradial half-section, a medium line of the tread band and the rotationaxis of the tyre form a predetermined angle different from zero, whereina median circumferential line of the tread band is axially shifted by apredetermined distance between about 3 mm and about 30 mm relative tothe mid-line plane of the tyre, wherein axial end portions of the treadband have different axial distances from the mid-line plane of the tyre,and wherein the axial end portion that is axially farthest from themid-line plane of the tyre is also the axial end portion that isradially the farthest from the rotation axis.
 2. The tyre as claimed inclaim 1, wherein said predetermined angle is greater than about 0.5°. 3.The tyre as claimed in claim 1, wherein said predetermined angle is lessthan about 5°.
 4. The tyre as claimed in claim 1, comprising asymmetricsidewalls relative to a mid-line plane.
 5. A motor-vehicle, comprising:at least one wheel mounted with a predetermined camber angle differentfrom zero, wherein the wheel comprises a rim and a tyre mounted on therim and inflated to an operating pressure, wherein, in each radialhalf-section, a corresponding medium line of a tread band and a rotationaxis of the tyre form a predetermined angle substantially equal to saidpredetermined camber angle, such that any medium line of the tread bandplaced in correspondence with a footprint area is substantially parallelto the ground, the predetermined angle being different from zero;wherein the rim has housings for beads, the beads being symmetric to amid-line plane of the wheel, wherein, in a radial half-section of thetyre, the tread band is symmetric relative to a symmetry axis thereof,wherein a median circumferential line of the tread band is axiallyshifted by a predetermined distance between about 3 mm and about 30 mmrelative to, the mid-line plane of the wheel, wherein axial end portionsof the tread band have different axial distances from the mid-line planeof the wheel, and wherein an axial end portion of the tread band that isaxially farthest from the mid-line plane is also the axial end portionthat is radially farthest from the rotation axis.
 6. The motor-vehicleas claimed in claim 5, wherein the camber angle is negative.
 7. Themotor-vehicle as claimed in claim 5, wherein the camber angle is betweenabout −0.5° and about −5°.
 8. The motor-vehicle as claimed in claim 5,wherein the median circumferential line is shifted toward an outsiderelative to the motor-vehicle.
 9. The motor-vehicle as claimed in claim5, wherein the tyre has asymmetric sidewalls relative to the mid-lineplane of the wheel.
 10. A wheel for motor-vehicles, comprising: a rimcomprising two housings for beads having a same radial distance from arotation axis of the wheel; and a tyre mounted on the rim inflated to anoperating pressure and comprising a tread band, wherein, in each radialhalf-section of the wheel, the tread band is symmetric relative to asymmetry axis thereof, and wherein, in each radial half-section, amedium line of the tread band and the rotation axis of the wheel form apredetermined angle different from zero, wherein the beads are symmetricto a mid-line plane of the wheel, wherein a median circumferential line,of the tread band is axially shifted by a predetermined distance betweenabout 3 mm and about 30 mm relative to a mid-line plane of the wheel,wherein axial end portions of the tread band have different axialdistances from the mid-line plane of the wheel, wherein the axial endportion of the tread band, that is axially farthest from the mid-lineplane of the wheel is also the axial end portion that is radiallyfarthest from the rotation axis.
 11. The wheel as claimed in claim 10,having asymmetric sidewalls relative to the mid-line plane of the wheel.